1. Technical Field of the Invention
The present invention generally relates to digital lightwave communications. More particularly, and not by way of any limitation, the present invention is directed to a system and method for generating reliable return-to-zero (RZ) optical data capable of long-haul transmission.
2. Description of Related Art
One of the most important characteristics of a lightwave transmission system is how large a distance can be spanned between a receiver and a transmitter while maintaining the integrity of the transmitted data. Such systems can be limited by the output power of the transmitter, the receiver performance characteristics, specifically receiver sensitivity, or both. The method of modulating the digital output from a transmitter can also greatly influence the distance separating the transmitter from the receiver. Modulating a digital lightwave output generates the digital “1”'s and digital “0”'s that are transmitted, and hence determines the content and integrity of the digital signal. From an economic viewpoint, the distance that can be spanned between a transmitter and a receiver, while maintaining data integrity, determines the expenditures that must be made to physically lay fiber in the ground or to install repeaters and other supporting equipment.
One way to control the output of a transmitter disposed in a digital lightwave communications system is to directly modulate the light source, e.g., a laser source. For example, the laser could be turned on and off at intervals, thus generating digital 1's (when the light source is on) and digital 0's (when the light source is off). This can be accomplished by turning the current to the laser on and off. While this method may work in lower speed applications, in high-speed digital lightwave communications it is not practical to directly modulate the output of the laser because, as the current to the laser is changed, the wavelengths of the laser outputs are also slightly changed.
Direct laser modulation could thus cause significant dispersion in each of the different wavelengths traveling along a fiber optic cable, resulting in noise and data corruption at the far end (i.e., receiver end) of a high-speed digital lightwave system. This is because, particularly in a directly modulated laser system, multiple wavelengths are introduced by the modulation process. Each of these wavelengths has a slightly different propagation time, resulting in overlap at the receiver and therefore in possible data corruption and/or loss. In systems employing wavelength division multiplexing (WDM) schemes, a significant amount of noise also results from carrying multiple wavelengths on a single fiber. This can result in loss of receiver sensitivity, because it is more difficult for the receiver to distinguish between the digital 1's and 0's, and hence to interpret the data carried by the signal.
Accordingly, current high-speed digital lightwave communications systems use modulators instead to modulate the laser output. Modulators do not affect the wavelengths carrying the data signal as much as direct modulation. However, these modulators require a data amplitude input (which data is typically in the range of one or more Gigabits per second (Gbps), i.e., in the radio frequency or RF range) and bias point that must be set and maintained at or near an optimum value for each modulator. Otherwise, the resulting wavelength shift in the transmitted data, along with the inherent noise and dispersion prevalent in lightwave transmission systems, can result in the signals received at the receiver being noisy and difficult to differentiate.
Furthermore, the return-to-zero (RZ) data format is generally preferred over the non-return-to-zero (NRZ) data format in high performance optical communications systems due to the better performance that RZ coding provides in the presence of noise. NRZ data refers to data output in which the data signal does not return to a zero value between data transitions. For example, as alluded to above, if the data output is a digital 0 followed by a digital 1, the light source within the optical transmission system transitions from “off” to “on”. However, if the next data bit in the sequence is also a digital 1, the light source remains “on” without transitioning to “off”. Two successive digital 1 outputs are thus seen as a continuous “on” period of the light source that is equal to two data intervals. The light source only returns to zero when the next data bit is itself a zero.
Whereas the NRZ data format is simple and inexpensive, it does not provide an optimal solution for long-haul, high-performance optical telecommunications systems. In particular, for example, where broadband (i.e., multi-channel) WDM systems that use optical amplifiers to increase signal performance are employed, noise in the optical signal is also enhanced thereby, which necessitates a higher resolution for reliable data transfer.
It is well known that in the presence of noise RZ coding provides better performance. RZ coding is an optical transmission format that provides a zero transition (i.e., the light source is off) between each data bit. In an RZ system, accordingly, the light source returns to an off condition for half the bit interval. In typical implementations, a clock signal associated with the data is also provided as an input to the modulator, which clock signal is used for blanking out a portion of the data intervals of the NRZ data. However, the phase relationship between the data and clock signal associated therewith must be optimally disposed such that the blanking operation is performed at appropriate times so as not to corrupt the data in the first place.
Current techniques that address this problem involve characterizing the various individual components disposed in the clock and RF data paths and then incorporate fixed temperature compensation in a phase shifter associated with the clock input. While such solutions may be sufficient in some applications, they are not satisfactory in high performance long-haul transmission systems. It should be appreciated that several deficiencies such as, for example, component variation over time, unit-to-unit variance in performance, thermal sensitivity, et cetera, cause the clock/data phase to uncontrollably drift from an initial set point, thereby degrading the reliability of transmitted data.